Technical Field
The present invention relates to an exhaust gas purifying catalyst and a method for producing the same.
Background Art
In a variety of industries, a variety of attempts has been made worldwide to reduce environmental impacts and burdens. In particular, in the automobile industry, development for promoting the spread of not only fuel-efficient gasoline engine vehicles, but also so-called eco-friendly vehicles, such as hybrid vehicles and electric vehicles, as well as for further improving the performance of such vehicles has been advanced day by day. In addition to the development of such eco-friendly vehicles, research about an exhaust gas purifying catalyst for purifying exhaust gas discharged from an engine has also been actively conducted.
An exhaust gas purifying catalyst contains an oxidation catalyst, a three-way catalyst, a NOx storage-reduction catalyst, and the like. A noble metal catalyst, such as platinum (Pt), palladium (Pd), or rhodium (Rh), exhibits catalytic activity in such exhaust gas purifying catalyst. The noble metal catalyst is typically used while being carried on a porous support made of a porous oxide, such as alumina (Al2O3).
A catalytic converter for purifying exhaust gas is typically arranged in an exhaust system for exhaust gas that connects a vehicle engine and a muffler. The engine may sometimes discharge environmentally harmful substances, such as CO, NOx, and unburned HC and VOC. In order to convert such harmful substances into allowable substances, exhaust gas is passed through a catalytic converter in which a catalyst layer, which contains a noble metal catalyst, such as Rh, Pd, or Pt carried on a porous support, is arranged on the cell wall surface of a substrate, so that CO is converted into CO2, and NOx is converted into N2 and O2, while VOC is burned to generate CO2 and H2O.
As a porous support on which a noble metal catalyst is carried, a ceria-zirconia composite oxide (which is also referred to as a CeO2—ZrO2 solid solution, a CZ material, and the like) can be used. This is also referred to as a promoter, and is an essential component of the aforementioned three-way catalyst for concurrently removing CO, NOx, and HC that are harmful components in the exhaust gas. Examples of the essential component of the promoter include CeO2.
CeO2 has a property that its oxidation number changes to Ce3+ or Ce4+, for example, depending on the partial pressure of oxygen in the exhaust gas to which CeO2 is exposed, and has a function of absorbing or releasing oxygen as well as a function of storing oxygen (OSC: Oxygen Storage Capacity) to compensate for the excess or deficiency of electric charges. In addition, CeO2 can absorb and mitigate fluctuations of the exhaust gas atmosphere and maintain the air/fuel ratio at a level around the theoretical air/fuel ratio in order to retain a purifying window of the three-way catalyst.
By the way, exhaust gas purifying catalysts have a problem in that they have deteriorated catalyst performance when exposed to a high temperature. Thus, an improvement in the heat resistance is desired.
The conventional exhaust gas purifying catalysts have been developed with a view to, in order to maintain the specific surface area of a porous support, which has a noble metal catalyst carried thereon, after endurance, avoid sintering of the porous support by forming a number of relatively large pores within the powder of the porous support.
However, when a number of relatively large pores are formed within powder of a porous support, the effect of suppressing sintering (aggregation) of a noble metal catalyst would decrease, and further, when the volume of the powder of the porous support is increased, another problem may occur such that the amount of the catalyst that can be applied to a monolith is limited. Meanwhile, when the number of pores is reduced, the specific surface area of the porous support will decrease, and it becomes difficult for the porous support to carry a noble metal catalyst. Thus, it is quite difficult to adjust the number of pores from the perspective of both carrying a noble metal catalyst and suppressing the aggregation of the noble metal catalyst after endurance.
By the way, Patent Document 1 relates to an exhaust gas purifying catalyst that includes a porous support and catalytic noble metal carried on the porous support, and discloses an exhaust gas purifying catalyst in which the porous support contains particles of an aluminum-cerium-zirconium composite oxide produced from an metal alkoxide, and the composition of the particles in terms of the molar ratio is in the range of Ce/Zr=1/3 to 3/1 and Al/(Ce+Zr)=2 to 10.
In the exhaust gas purifying catalyst described in Patent Document 1, as the composition of the particles in terms of the molar ratio is adjusted in the range of Ce/Zr=1/3 to 3/1 and Al/(Ce+Zr)=2 to 10, OSC will hardly decrease even after endurance. Thus, a time for which the catalyst is exposed to a lean atmosphere is reduced, deterioration due to sintering of a noble metal catalyst and the like is suppressed, and the initial high activity can thus be maintained for a long time.
However, there is still room for improvement of the effect of suppressing deterioration due to sintering of a noble metal catalyst, more particularly, the effect of suppressing deterioration due to aggregation of a noble metal catalyst that would occur during endurance at a high temperature.